Such components are known in various embodiments, in particular as light-emitting organic components. One type of light-emitting organic component is organic light-emitting diodes (OLEDs). Since the demonstration of low operating voltages by Tang et al. (cf. C. W. Tang et al., Appl. Phys. Lett. 51 (12), 913 (1987)), organic light-emitting diodes have been promising candidates for producing new lighting and display elements. All of these components comprise a sequence of thin layers of organic materials, which are preferably applied in vacuo by vapour deposition or processed in their polymeric or oligomeric form in solution. After electrical contacting by means of metal layers, as a result of which contacts are produced, they form a wide range of electronic or optoelectronic components, for example diodes, light-emitting diodes, photodiodes, transistors and gas sensors, which in terms of their properties compete with the established components based on inorganic layers.
In the case of organic light-emitting diodes, light is generated by the injection of charge carriers, namely electrons from an electrode and holes from a counterelectrode or vice versa, into an arrangement of organic layers arranged therebetween, as a result of an externally applied voltage, the subsequent formation of excitons (electron/hole pairs) in an active zone and the recombination of these excitons in an emission zone to produce light, and said light is emitted from the light-emitting diode.
The advantage of such organic-based components over conventional inorganic-based components, for example semiconductors such as silicon or gallium arsenide, lies in the fact that it is possible to produce elements with a very large surface area, that is to say large display elements (display panels, screens). The organic starting materials are relatively inexpensive compared to inorganic materials. Moreover, due to their low processing temperature compared to inorganic materials, these materials can be applied to flexible substrates, which opens up a large number of new applications in the field of display and lighting technology.
U.S. Pat. No. 5,093,698 describes an organic light-emitting diode of the PIN type (PIN-OLED), which is an organic light-emitting diode with doped charge carrier transport layers. In particular, use is made of three organic layers which are located between two electrodes. In said document, n-doped and p-doped layers improve the injection of charge carriers and the transport of holes and electrons into the respectively doped layer. The proposed structure consists of at least three layers comprising at least five materials.
The energy levels HOMO (“Highest Occupied Molecular Orbital”) and LUMO (“Lowest Unoccupied Molecular Orbital”) are preferably selected such that both types of charge carrier are “trapped” in the emission zone so as to ensure efficient recombination of electrons and holes. The restriction of the charge carriers to the emission zone is achieved by suitably selecting the ionization potentials or electron affinities for the emission layer and/or the charge carrier transport layer, which will be discussed in more detail below.
One prerequisite for generating visible light is that the excitons formed in the recombination zone have at least an energy that corresponds to the wavelength of the light to be emitted. Here, the highest energies are required to produce blue light, which has a wavelength in the range from 400 to 475 nm. In order to facilitate the injection of charge carriers from the charge transport layers into the emission zone, it is advantageous to use as matrix materials for these layers preferably materials which are adapted in terms of their energy levels to the emission zone such that there is the highest possible energy difference between the level of the electrons in the electron transport layer and the level of the holes in the hole transport layer.
The mode of operation of light-emitting components does not differ from the mode of operation of components which emit electromagnetic radiation close to the visible spectral range, for example infrared or ultraviolet radiation.
In the case of materials for electron transport layers, experiments have found that the energy of the LUMO should be at most −2.7 eV or less. This corresponds to a value of at most −2.1 V vs. Fc/Fc+ (vs. ferrocene/ferrocenium). Standard materials which are used as materials for electron transport layers in OLEDs have such a LUMO, for example BPhen (LUMO −2.33 eV) and Alq3 (LUMO −2.47 eV). In the case of materials for hole transport layers, the HOMO is preferably −4.8 eV or less, corresponding to 0 V vs. Fc/Fc+ (vs. ferrocene/ferrocenium) or more.
In order to achieve a suitable doping effect, the p-dopants and n-dopants must have certain reduction potentials/oxidation potentials in order to obtain an oxidation of the matrix of the hole transport layer so as to achieve p-doping and a reduction of the matrix of the electron transport layer so as to achieve n-doping.
To determine an ionization potential, ultraviolet photoelectron spectroscopy (UPS) is the preferred method (cf. R. Schlaf et al., J. Phys. Chem. B 103, 2984 (1999)). Ionization potentials are usually determined in the solid state. However, it is in principle also possible to determine the gas ionization potentials. However, the measured values obtained by means of the two different methods differ on account of interactions which occur in the solid. One example of such an effect due to an interaction is the polarization energy of a hole which is produced as a result of photoionization (N. Sato et al., J. Chem. Soc. Faraday Trans. 2, 77, 1621 (1981)). The ionization potential corresponds to the point at which photoemission begins on the flank of high kinetic energies, i.e. the weakest bound photoelectron.
A related method, inverse photoelectron spectroscopy (IPES), is used to determine electron affinities (cf. W. Gao et al., Appl. Phys. Lett. 82, 4815 (2003)), but this is a less established method. Alternatively, the solid potentials can be estimated by means of electrochemical measurements of oxidation potentials Eox and reduction potentials Ered in the solution, for example by means of cyclic voltammetry (CV) (cf. J. D. Anderson, J. Amer. Chem. Soc. 120, 9646 (1998)).
Empirical formulae for converting the electrochemical voltage scale (oxidation potentials) into the physical (absolute) energy scale (ionization potentials) are known (cf. for example B. W. Andrade et al., Org. Electron. 6, 11 (2005); T. B. Tang, J. Appl. Phys. 59, 5 (1986); V. D. Parker, J. Amer. Chem. Soc. 96, 5656 (1974); L. L. Miller, J. Org. Chem. 37, 916 (1972); Y. Fu et al., J. Amer. Chem. Soc. 127, 7227 (2005)). No correlation between reduction potential and electron affinity is known, since electron affinities can be measured only with difficulty. For the sake of simplicity, therefore, the electrochemical and physical energy scale are converted into one another using IP=4.8 eV+e*Eox (vs. ferrocene/ferrocenium) and EA=4.8 eV+e*Ered (vs. ferrocene/ferrocenium) (cf. B. W. Andrade, Org. Electron. 6, 11 (2005)). The conversion of various standard potentials or redox pairs is described for example in A. J. Bard et al., “Electrochemical Methods: Fundamentals and Applications”, Wiley, 2nd edition 2000. Information concerning the influence of the solvent used for the measurement can be found in N. G. Connelly et al., Chem. Rev., 96, 877 (1996).
It is customary to use the terms ionization potential and electron affinity synonymously with the terms energy (or energy layer) of the HOMO and energy (or energy layer) of the LUMO (Koopman's theory). It should be noted here that the ionization potential/electron affinity are stated in such a way that larger values signify stronger binding of the released/attached electron to the respective molecule. The energy scale of the molecular orbitals (e.g. HOMO or LUMO) is measured the other way round.
The potentials given in the present application relate to the value in the solid.
U.S. Pat. No. 5,093,698 discloses a component structure for an OLED which leads to a greatly improved charge carrier injection from the electrodes into the organic layers. This effect is based on considerable band bending of the energy levels in the organic layer at the interface to the electrodes (J. Blochwitz et al., Org. Electronics 2, 97 (2001)), as a result of which injection of charge carriers on the basis of a tunnel mechanism is possible. The high conductivity of the doped layers also prevents the voltage drop which occurs there during operation of the OLED.
The injection barriers which may occur in OLEDs between the electrodes and the charge carrier transport layers are one of the main causes for an increase in the operating voltage compared to the thermodynamically justified minimum operating voltages. For this reason, many attempts have been made to reduce the injection barriers, for example by using cathode materials with a low work function, for example metals such as calcium, magnesium or barium. However, these materials are highly reactive, difficult to process and are only suitable to a limited extent as electrode materials. Moreover, any reduction in operating voltage brought about by using such cathodes is only partial.
A further possibility for improving the injection of electrons from the cathode into the electron transport layer consists in using LiF or other lithium compounds which are incorporated as a thin layer between an aluminium cathode and the electron transport layer. It is assumed that lithium, which has a lower work function than aluminium, is formed in the process (M. Matsumura et al., Appl. Phys. Lett., 2872, (1998)). However, this method functions only when using aluminium as the cathode material. Moreover, precise control of the layer thickness for the LiF layer is necessary, since only very thin layers in the region of a few nanometers give rise to the desired effect. The method also does not function in a satisfactory manner for inverted structures, in which the cathode is deposited first, followed by the organic layer sequence.
Another possibility for improved injection from the cathode into the electron transport layer is known (cf. Bloom et al., J. Phys. Chem., 2933, (2003)), in which organometallic complexes with a low work function are incorporated as a thin layer between the cathode and the electron transport layer in OLEDs.
As anode material, in OLEDs use is usually made of materials with a relatively high work function. By way of example, use is made of transparent conductive oxides, for example indium tin oxide (ITO) or indium zinc oxide (IZO). Attempts have also been made to improve the injection of holes from ITO anodes into the hole transport layers of OLEDs. By way of example, the work function of ITO can be increased by means of targeted treatment of the ITO surface with oxygen plasma (M. Ishii et al., J. Lumin., 1165, (2000)).
Furthermore, the documents U.S. Pat. No. 6,720,573 B2 and US 2004/0113547 A1 propose the use of substituted hexaazatriphenylenes as a layer for hole injection and/or hole transport, as a result of which the injection barrier between the anode and the hole transport layer is reduced. Document US 2004/0113547 A1 suggests a “virtual electrode” which forms when using an injection layer made from hexaazatriphenylenes. It is assumed here that the material in the injection layer has a higher stability in the reduced state than in the neutral state. Furthermore, the material has a low electron mobility and a high hole mobility. It is proposed in document US 2004/0113547 A1 that free electrons from the anode material are given to the hexaazatriphenylene layer, as a result of which the modules of this layer are partially reduced. Since the electrons exhibit a low mobility in the material, they remain tight against the interface to the anode, where they form a virtual cathode. When a voltage is applied, an injection of holes from the anode into the hexaazatriphenylene layer is made easier as a result of the “virtual cathode”, i.e. the negative charges, immediately next to the anode. Due to the high hole mobility of this layer, the holes then quickly continue to migrate in the direction of the actual cathode.
It is further stated in document US 2004/0113547 A1 that this injection layer leads to good results in particular with anode materials having a low work function. This finding goes against the customary attempts to use anode materials with a high work function. However, this obviously represents a restriction with regard to the usability of these materials.
Proposals are thus known both for charge carrier injection from the anode and from the cathode into the charge carrier transport layers of OLEDs. However, these are only partially able to solve the existing problems concerning injection. In particular, it is not guaranteed that the injection of charge carriers into the charge carrier transport layers will take place largely without barriers, as is the case in PIN-OLEDs.
PIN-OLED technology will be discussed in more detail below.
Furthermore, the transport of the charge carriers is a possible source of an undesired voltage drop. In undoped layers, charge transport takes place according to the theory of space charge-limited currents (cf. M. A. Lampert, Rep. Progr. Phys. 27, 329 (1964)). Here, the voltage necessary to maintain a certain current density increases as the layer thickness increases and as the charge carrier mobility decreases. Organic semiconductor materials these days have high charge carrier mobilities of more than 10−5 cm2/Vs, but these are often not sufficient to ensure a charge carrier transport that is largely free of voltage losses at increased current densities such as those necessary in the operation of OLEDs with high luminances. Compared with this, a minimum layer thickness for the transport layer thicknesses must be adhered to, in order for example to avoid short-circuits between the electrodes and quenching of luminescence at the metal contacts.
For PIN-OLEDs, the conductivities of the doped layer are up to five orders of magnitude or more higher than undoped layers. The layer behaves like an ohmic conductor, as a result of which a voltage drop over the (doped) charge carrier transport layers is very low even when operating OLEDs with high current densities. With a conductivity of 10−5 S/cm for example, a voltage of 0.1 V drops over a doped organic charge carrier transport layer with a thickness of 100 nm at a current of 100 mA/cm2. By contrast, in the case of an undoped charge carrier transport layer (space charge limitation of the current) with a mobility of 10−5 cm2/Vs, a voltage of 5.4 V is required for this current density.
In document DE 100 58 578 C2, blocking layers were inserted between the central emission layer and at least one charge carrier transport layer. Here, the charge carrier transport layers are likewise doped with either acceptors or donors. It is described how the energy levels of the blocking materials must be selected in such a way as to restrict electrons and holes in the light-emitting zone, i.e. to prevent the charge carriers from leaving the emission zone by means of diffusion. Therefore, the known structure actually permits high efficiencies since the additional intermediate layers also act as a buffer zone for previously possible quenching effects at dopant impurity sites.
Cancellation of luminescence may be brought about by a number of effects. One possible mechanism is known as exciplex formation. In such a case, holes and electrons which are actually intended to recombine with one another on an emitter molecule in the emission zone are located on two different molecules at one of the interfaces to the emission layer. This so-called exciplex state can be understood as a charge transfer exciton, with the molecules involved being of different nature. In the event of an unsuitable choice of materials for the block and emission layer, this exciplex is the lowest possible excited state in terms of energy, so that the energy of the actually desired exciton can be transmitted to an emitter molecule in this exciplex state. This leads to a reduction in the quantum yield of the electroluminescence and thus of the OLED. This is associated with an electroluminescence of the exciplex which is shifted towards red, but which is then usually characterized by very low quantum yields.
Further mechanisms for luminescence cancellation which occur in OLEDs arise as a result of the interaction of excitons with charged or uncharged doping molecules on the one hand and/or with charge carriers on the other hand. The first mechanism is effectively suppressed by using undoped blocking layers due to the short range (for example <10 nm) of the interaction. Since charge carriers necessarily have to occur in and close to the emission zone during operation of the OLED, optimization can take place here only to the effect that an accumulation of charge carriers for example at a band discontinuity is prevented. This places particular requirements on the choice of band levels for the blocking material and the emitter, in order to prevent barriers for charge carrier injection and thus an accumulation of charge carriers.
In addition to the above-described effects of increasing the OLED efficiency by suppressing non-emitting quenching processes of the excitons, the use of intermediate layers at the interface between the transport layers and the emission zone can also fulfil the purpose of facilitating charge carrier injection into the emission zone. In order to produce visible light, it is necessary to generate excitons on the emitter molecules which have at least an energy corresponding to the wavelength of the light emitted by the OLED. This energy often corresponds to a value greater than the difference in level defined by the difference between the HOMO of the hole transport layer and the LUMO of the electron transport layer. In order then to prevent excessively high injection barriers to injection from the charge carrier transport layers into the emission zone, which could lead to an increased charge carrier density at this interface and to the formation of a space charge zone, it is often advantageous to introduce additional thin layers with thickness in the range of a few nanometers between the charge carrier transport layers and the emission zone. These should then lie, in terms of their energy, with the level of their HOMO (in the case of the hole transport layer) or their LUMO (in the case of the electron transport layer) between the charge carrier transport layers and the level of the emission zone, as a result of which the charge carrier transport is facilitated and the formation of a space charge zone is prevented.
With suitable layer arrangements, OLEDs based on PIN technology achieve very high current efficiencies while simultaneously having very low operating voltages, as a result of which it is possible to achieve extremely high performance efficiencies of more than 100 lm/W (J. Birnstock et al., IDW, Proceedings, S. 1265-1268 (2004)), which have to date not been possible with alternative technologies.
However, the production of doped charge carrier transport layers represents an additional technological obstacle in the production of an OLED. Besides the emission zone, which is often constructed by the simultaneous evaporation of two or more materials in one or more layers, for PIN OLEDs it is additionally necessary to produce the transport layers from two materials in each case. To this end, therefore, two evaporation sources are required, which have to be heated and controlled separately, which is naturally associated with a more complicated and therefore more expensive design of the production installation. Other methods for producing the OLED layers, for example by growing the layers on from a carrier gas which is loaded with the OLED materials, are also more complicated with regard to simultaneous deposition. For such methods, it must be ensured in particular that the evaporation temperatures of the materials to be deposited are advantageously as close to one another as possible, so as to prevent the possible deposition of the less volatile substance at cooler parts in the production installation. However, if the evaporation temperatures are too far apart, and thus the parts of the installation which come into contact with the gas stream have to be brought to suitably high temperatures so as to prevent such a deposition, there is a risk of chemical decomposition of the more volatile component at the hot vessel walls. Moreover, in the case of the doping of charge transport layers, it is also conceivable that a reaction between the transport matrix and the dopant may occur already in the gas phase.
Document WO 2005/086251 deals with the use of a metal complex as n-dopant for an organic semiconductive matrix material, an organic semiconductor material and an electronic component and also as dopant and ligand.
One of the main fields of application for organic light-emitting diodes is in display technology. Both in the field of passive matrix display elements and in the field of active matrix display elements, OLEDs have in recent years obtained an increasing market share, with the price pressure naturally being high. To this end, the increased technological manufacturing outlay in the production of PIN OLED components must be weighed up against the improved performance characteristic compared to conventional OLEDs, which means that under some circumstances the commercial success of PIN technology may be impaired.